Add __enter__ and __exit__ methods to Solver class in Python API

[alperaltuntas]

This pull request adds the __enter__() and __exit__() methods to the Solver class in Python API for implicit scope management. This allows the usage of the with statement as a more concise and safer alternative to explicitly calling the push() and pop() methods when appropriate.

Example usage:

from z3 import *

p, q = Bools('p q')

with Solver() as s:
    s.add(Implies(p, q))
    s.add(Not(q))
    with s:
        s.add(p)
        print(s.check()) # unsat
    print(s.check())     # sat

print(s.assertions())    # []